1. Field of the Invention
The present invention relates generally to tools for use in nuclear reactor facilities and, in particular, to a tool and method for exchanging and shuffling control rods in a boiling water nuclear reactor.
2. Description of the Prior Art
In boiling water reactors (BWRs) control rods are used to control the reactivity and power output of fuel assemblies. The control rods are used to shut down the reactor and compensate for long term reactivity changes associated with uranium depletion and fission product buildup. The control rods also function to control the power distribution among fuel assemblies. The control rods enter the core of the reactor vessel from the bottom, and their presence in the lower portion of the core helps to balance the reactivity effects of steam voids in the upper portion of the core and thus tends to flatten the axial power distribution.
The basic design of the BWR control rod assembly is shown in FIG. 1. The control rod assembly 20 consists of a cruciform-shaped stainless steel sheath 21 enclosing eighteen to twenty-one absorber rods 22 in each of its four wings. The absorber rods 22 are stainless steel tubes filled with boron carbide powder (B.sub.4 C) compacted to approximately 65% of theoretical density in which the B.sup.10 isotope is the primary neutron absorber. The tubes are seal-welded with end plugs on either end. The individual tubes act as pressure vessels to contain the helium gas released by the boron-neutron capture reaction.
A control cell in which the control rod 22 is positioned is shown schematically in FIGS. 2a-c and 3. The control cell includes a control rod guide tube 24, a fuel support piece 25, and fuel assemblies 26. A plurality of the control rod guide tubes 24 are inserted into holes arranged in a grid in the lower core plate 27. Each guide tube 24 is oriented by alignment lugs 28 which fit over the anti-rotation pin 29 in the lower core plate 27. The control rod drive 30 penetrates the reactor pressure vessel bottom head and is latched to a coupling 31 on the lower end of the guide tube 24. The control rod drive piston 32 enters the lower end of the guide tube 24 and is coupled to the control rod 20 with a spud mechanism 33. A velocity limiter 34 is formed at the lower end of the control rod 20. Guide rollers 35 on the velocity limiter 34 bear on the inside wall of the guide tube 24 to give lateral support for the control rod 20.
The fuel support piece 25 rests in the top end of the guide tube 24. The fuel support piece 25 is supported by a 45 degree sealing ring on the top inside diameter of the guide tube 24. The orientation of the fuel support piece 25 is maintained by alignment lugs 28 on one corner which engage the anti-rotation pin 29 in the lower core plate 27. The fuel support piece 25 is cylindrical at its lower end to fit into the guide tube 24. The upper portion of the fuel support piece 25 consists of four lobes which hold the four fuel assemblies 26 of the control cell. Beneath the hole in the upper surface of each lobe is a cast-in flow path which directs coolant flow into the fuel assembly 26. The coolant flow enters through orifices 36 in the outside diameter of the lower portion of the fuel support piece 25, which are aligned with holes in the control rod guide tube 24. The fuel support pieces 25 for peripheral control cells have smaller orifices 36 for the fuel bundle positions that are on the outer row of fuel. This orificing arrangement makes the peripheral fuel support pieces 25 unique to their location so that they cannot be moved from place to place during control rod change operations. The control rod 20 passes through a cross-shaped hole 37 in the fuel support piece 25.
Also shown in FIG. 3 is a neutron source 38 which occupies a blind corner hole and a bypass flow plug 39 positioned in a through hole adjacent to the fuel support piece 25.
The four fuel assemblies 26 of the control cell are supported by the lobes of the fuel support piece 25 and occupy the corners of the cell leaving a cross-shaped space at the center for the control rod 20. The rollers 35 on the tip of the control rod 20 contact the outer surface of the fuel assembly channel to provide lateral support for the control rod 20 when it is inserted.
In order to replace a control rod 20 according to the conventional procedure, the fuel assemblies 26 and the fuel support piece 25 in its control cell must be removed. The control rod 20 can then be uncoupled from the drive mechanism 30 and removed from above the reactor.
The design life of the control rods 20 requires that the rods be replaced on a regular basis. In addition to replacing the control rods 20, some plants have found it advantageous to extend the life of the control rods 20 by shuffling the control rods 20 between high and low exposure locations in the core in order to equalize the exposure over a larger number of rods. Some reactors have adopted a particular fuel loading scheme, which concentrates the exposure on one group of control rods 20 (e.g., 1/6 of the total number of rods) while keeping the remainder of the control rods in a fully withdrawn position. This loading scheme requires more frequent control rod shuffles in order to equalize exposure among the blades of the control rods.
Movements of fuel assemblies 26, control rods 20, and associated components are accomplished through use of a refueling platform or bridge and its associated hoisting equipment. A conventional arrangement is shown in FIGS. 4a and 4b. The refueling platform 40 spans the reactor vessel 41 and the reactor cavity 42 and the fuel pool 43. The refueling platform 40 is supported at its ends by respective A-frame supports 44 which run on tracks 45 embedded in the floor. A trolley 46 runs across the platform 40 on its upper structure at a level several feet above the tracks 45.
A first main hoist 46a is supported by the trolley 46 and has a lift cable 47a extending therefrom for raising and lowering the fuel assemblies 26 or a blade guide 49, for example. A second hoist 46b is mounted to the frame of the trolley 46 and has a lift cable 47b for raising and lowering the fuel support piece 25, for example. A third hoist 46c is mounted to a monorail extending along the platform 40 and has a lift cable 47c for raising and lowering the control rod 20, for example. The platform 40 and trolley 46 are designed to transport fuel assemblies 26, control rods 20, blade guides 49, fuel support pieces 25, and other contaminated components under water between various points in the reactor cavity 41 and the reactor vessel 42, or between the reactor and the spent fuel pool 43. A suitable grapple 50a, 50b, 50c is secured to the lower end of each of the lift cables 47a, 47b, 47c, respectively, for attaching the lift cables to the components to be lifted.
A typical sequence of moves involved in changing a control rod 20 using conventional tools will now be described with reference to FIGS. 4a-b and 5 of the drawings.
With the control rod 20 fully inserted in the control cell, two diagonally opposed fuel assemblies 26 are removed one at a time from the control cell using a main grapple 50a supported by the lift cable 47a of the main hoist 46a and transported to storage in the spent fuel pool 43, to an in-vessel storage rack 48, or to another core location as part of a fuel shuffle. A blade guide 49 is transported on the main grapple 50a of the lift cable 47a from a storage location and inserted into the open holes of the control cell left by the removed fuel assemblies 26. The remaining two fuel assemblies 26 are then removed one at a time from the control cell using the main grapple 50a and hoist 46a and are stored in the fuel pool 43 or other suitable location.
The control rod 20 is then fully withdrawn from the cell using the control rod drive mechanism 30, and the drive is then valved out of operation. The blade guide 49 is then removed from the control cell using the main grapple 50a and hoist 46a and taken to a storage location or left hanging from the lift cable 47a. This is represented by step (a) in FIG. 5.
A fuel support grapple 50b and grid guide are then installed on the lift cable 47b extending from the second hoist 46b. The arms of the grapple 50b are closed to allow the grapple 50b to pass through the top grid 51. After the grapple 50b has cleared the top grid 51, the grapple arms are released and the grapple 50b is lowered onto the fuel support piece 25. When the grapple 50b is correctly seated on the fuel support piece 25, the grapple 50b closes on the fuel support piece 25 and is then lifted free of the control rod guide tube 24 with the second hoist 46b. The fuel support piece 25 is then raised up through the top grid 51 and left suspended from the lift cable 47b of the second hoist 46b. This is represented by step (b) in FIG. 5.
The control rod 20 is then unlatched from the control rod drive 30 and removed using a control rod latch tool (not shown) supported by the third hoist 46c. A grid guide may be used with the latch tool. The control rod latch tool is lowered onto the top handle 52 of the control rod 20 with its lower section resting in the angle between two of the wings of the control rod 20.
When the latch tool is fully seated on the control rod 20, actuation of the tool with the engage button on the hoist pendant causes the lifting hook to engage the top handle 52 of the control rod 20 and the actuator hook in the lower end of the tool to engage and lift the unlatching handle 53 of the control rod 20, thus releasing the control rod 20 from the control rod drive 30. The control rod 20 is then lifted out of the guide tube 24 with the latch tool. As the control rod 20 is lifted, the position of the control rod drive 30 is monitored to verify that the control rod 20 has been successfully unlatched from the drive. The control rod 20 is then transported to the fuel storage pool 43 and placed in a storage rack 54. This is represented by step (c) in FIG. 5.
A new control rod 20 is then transported from the pool 43 with the control rod grapple 50c suspended from the third hoist 46c. This is represented by step (d) in FIG. 5. The new control rod 20 is placed in the control rod guide tube 24 and lowered to rest on the bottom of the guide tube 24. Coupling of the control rod 20 to the drive 30 is usually deferred until after the fuel support piece 25 and blade guide 49 have been inserted into the cell.
The fuel support piece 25 is then lowered into the control cell and over the top of the control rod 20 until it is seated in the top of the guide tube 24. This is represented by step (e) in FIG. 5. When the fuel support piece 25 is installed in the proper orientation, the grapple arms are released and the grapple 50b is lifted off of the fuel support piece 25.
The blade guide 49 is then installed in the control cell from its hanging position or its storage position. This is represented by step (f) in FIG. 5.
The control rod 20 is then recoupled by moving the piston 32 of the control rod drive 30 to a partially inserted position. The control rod 20 is then raised to its fully inserted position, and two fuel assemblies 26 are installed in the positions not occupied by the blade guide 49. The blade guide 49 is then removed, and two more fuel assemblies 26 are installed in the positions from which the blade guide 49 was removed.
Control rod replacement and shuffling operations at BWRs using the conventional procedures and tools described above often require excessively long times to perform. A wide variation in the time required (e.g., from 2 to 10 hours per control rod) and frequent long delays for serious problems (e.g., jammed fuel supports or inoperable tools) are common. Thus, there is a need for improved procedures and equipment to replace and shuffle control rods to minimize delays, to reduce average operation times, and to ensure safe operating conditions.